1. Field of the Invention
The present invention relates to an improved expansion assembly for mine roof bolts used in relatively-small diameter holes and, more particularly, to expansion assemblies that can be used with or without resin-bonding materials.
2. Brief Description of the Prior Art
Examples of four-prong mine roof bolt expansion shell assemblies used in one-inch diameter mine roof bore holes are disclosed in U.S. Pat. Nos. 4,904,123; 4,969,778; and 5,078,547, all herein incorporated by reference in their entirety and all assigned to the assignee of the present invention.
In general, four-prong mine roof bolt expansion shell assemblies include a plug, which is attached to a mine roof bolt in mine roof support applications, and a four-prong expansion shell that slidingly engages the plug.
In one commercial embodiment, the plug has a height of approximately 1{fraction (3/16)} inches, an outside diameter of approximately 0.9 inches, and is made from ASTM A220 Grade 50005 pearlitic malleable iron. The plug has approximately 6.5 degrees of side taper, with the length of taper being approximately one inch. Internally-defined threads are provided for attaching the plug to a mine roof bolt, wherein the threads are generally ⅝ inch, 11 per ASTM F432-95.
The plug defines four resin grooves spaced ninety degrees apart with respect to each other, with each resin groove being approximately 0.074 inch deep and approximately 0.268 inch wide. The plug further defines a number twelve through hole that receives a wooden shear pin which acts as a delay mechanism. The total area of resin grooves is approximately 0.040 square inch, and the total area for resin flow in a substantially one-inch diameter bore hole is approximately 0.117 square inch.
The four-prong expansion shell generally has four prongs and an inside square taper leave. The four-prong expansion shell is preferably made from ASTM A47 Grade 32510 ferritic malleable iron, has a height of approximately 2{fraction (11/32)} inches, and an outside diameter of approximately {fraction (15/16)} inch. The degree of inside taper leave is approximately 6.5 degrees, and the length of taper is approximately {fraction (39/64)} inch. The four prongs define four grooves spaced ninety degrees apart, with each groove width being approximately xc2xc inch and each groove length being approximately 2{fraction (1/16)} inches. The inside diameter of the four-prong expansion shell is approximately {fraction (21/32)} inch. Each of the four prongs define approximately eight total serrations spaced approximately {fraction (3/16)} inch apart with respect to one another, with three serrations at ten degrees and five serrations at twenty degrees. The total serrated surface area of all of the four prongs is 3.483 square inches.
Given the fact that the bore hole diameter is fixed in small bore applications to approximately one inch, there is little flexibility with respect to the diameter of the expansion shell. However, there is an ever present need to secure small diameter bore hole mine roof bolts in mine roofs such that the bolts will resist higher stress loads.
One embodiment of the present invention generally includes an expansion assembly configured to be attached to a mine roof bolt. The expansion assembly may include a plug defining an interior cavity and an outer surface and an expansion shell having a plurality of spaced-apart prongs and defining a plurality of shell grooves, wherein each of the spaced-apart prongs is oriented diametrically opposed to a corresponding shell groove. In one configuration, the expansion shell is positioned adjacent to the outer surface of the plug and is slideably movable with respect to the plug.
The plug may generally define an internal cavity, define threads in the internal cavity, define three resin grooves spaced about 120 degrees apart, and may also define a side extension that extends along a length of the plug. The expansion shell may define only three prongs also spaced approximately 120 degrees apart and three shell grooves, and may further comprises a ring, with the three prongs integrally-formed with the ring. The expansion shell may also define eight spaced-apart serrations, with three of the serrations angled in one orientation and five of the serrations angled at a second orientation.
The present invention helps to increase the stress load resistance of small diameter mine roof bolts by providing an expansion shell assembly for small diameter bore holes, particularly one-inch diameter mine roof bore holes, wherein the plug has three resin grooves, and the expansion shell has three prongs.
The three-prong design increases the amount of surface area for resin flow, increases the total external area of the three-prong expansion shell which permits the three-prong expansion shell to set or grab the walls of the bore hole quicker (allowing a smaller support nut to be used), and is less expensive to manufacture.
These and other advantages of the present invention will be clarified in the description of the preferred embodiment taken together with the attached drawings in which like reference numerals represent like elements throughout.